No, I don't think there is much of a demand, at least not enough to sustain an entire business. You would need to be pulling in serious money from publishers, and on top of that, you're trying to change a system that largely already works. I've published myself, and e-mailing drafts was fine. The only times I've ever seen collaborative documents being used by average people were a) real-time coding interviews, and b) filling out forms. People would not be willing to pay money in either case, no matter how good it is.